It depends on how you're at the glory hole:
- If you've seen a nice cock sticking through a glory hole and you're going to suck it and immediately after they've left someone sticks their dick through then the polite thing to do (and the right thing as a cocksucking faggot) would be to suck that dick too. If you don't want to, or you've got plans for something else, then you only got down there to suck the dick you saw so you could justify not sucking others. Can't really justify not sucking that dick yet staying at the glory hole though - if you're not wanting to suck this new dick then you should leave, let someone else suck it.
- If however you've gotten down to an empty gloryhole looking to suck cock then you shouldn't refuse anything. You've put yourself in that position, you're advertising that you're going to suck dick. The dick shouldn't matter because that is not your place to decide or choose. It's like a hotel refusing to serve gay couple - you're there to provide a service and you do not get to discriminate. Only difference is your service is your mouth and cunt for men to use.